Effective treatment of a patient during a medical emergency requires that care of the patient begin immediately at the scene of the emergency. However, before such care is administered to the patient, an assessment must be made of a patient's status. A patient's status is normally diagnosed by visual examination and measurement of one or more traditional vital signs to determine cardiovascular and pulmonary functions. Cardiovascular functions include blood pressure, heart rate, ECG, and capillary refill time. Pulmonary functions include respiration rate and respiration volume.
The speed of assessing a patient's status determines the immediacy and type of medical care for that patient, which, in turn, determines the ultimate survival of such a patient. In the case of multiple patients, the assessment of each patient's status determines the priority of care to be administered to any one of the multiple patients, but also causes a delay in the administration of care while identifying patients who do not require immediate care or are beyond medical help.
Tissue hypoxia is a major cause of morbidity and the ultimate cause of death in such patients. A primary cause of tissue hypoxia is a failure of peripheral oxygen delivery to the tissue of a patient. Oxygen delivery depends on the patient's external oxygen supply, pulmonary function, cardiac output, and the oxygen transport capacity of the patient's blood. The oxygen transport capacity of blood, in turn, depends upon the concentration of functioning hemoglobin and the affinity of the hemoglobin for oxygen. Traditionally, the delivery of oxygen to the tissue (tissue oxygenation or tissue perfusion) is determined with noninvasive vital signs measurements or from invasive blood sampling and analysis of blood gases. However, blood sampling and analysis is inappropriate at the scene of an emergency. Vital signs measurements, while appropriate for monitoring a patient at the scene of an emergency, is slow. Typically, such measurements are on the order of one to two minutes. Sensors must be applied, vital signs determined, and the sensors removed. With multiple patients and limited medical personnel, determining these measurements delays the onset of care to each patient.
The amount of oxygen available for peripheral tissue oxygenation is best indicated by the amount of oxygenated hemoglobin. Oximetry is a photometric technique for determining the percentage of hemoglobin which is oxygenated in a tissue sample, either in vitro or in situ. Oximetry relies on unique spectral characteristics of oxygenated and deoxygenated hemoglobin moieties.
Oximetry does not measure absolute hemoglobin concentration, but measures the relative amount of oxygenated hemoglobin. In the case of reduced hematocrit, oxygen extraction from the blood will be accelerated and thus arteriolar and capillary hemoglobin saturation will be reduced and the oximeter will indicate a low reading. Under these conditions the reading can underestimate the correct SaO.sub.2 at values below 50% saturation, which is exacerbated by reduced hematocrit. However, the clinical meaning of such an underestimated reading is correct, because a percent saturation below 70% is outside of the normal clinical range and indicates life-threatening hypoxia. The oximeter will not overestimate SaO.sub.2.
Oximetry can be implemented either in a transmission mode or in a reflectance mode. In transmission mode oximetry, a tissue sample is transilluminated and the intensity of specific wavelengths of light transmitted through the tissue sample is measured to determine the percentage of oxygenated hemoglobin. In reflectance mode oximetry, the tissue sample is illuminated and the intensity of specific wavelengths of backscattered light is measured to determine the percentage of oxygenated hemoglobin.
Wood et al. [Photoelectric Determination of Arterial Oxygen Saturation in Man, J. Lab. Clin. Med., 34:387-401 (1949)] were the first to develop a device for absolute SaO.sub.2 measurement in vivo. Wood et al. developed a transmission mode ear oximeter using red and infrared light. This oximeter yielded results within a few percentage points of direct gaseometric analysis. However, fundamental problems exist with this approach, and with all later non-pulsatile, transmission oximeters. The procedure is slow (on the order of minutes), bloodless ear readings are required for correction of a nonhemoglobin-related transmission offset signal, and the pinna must be warmed to arterialize the blood.
Sutterer et al. [Calculation and Digital Display of Whole Blood Oxygen Saturation by Analog Techniques, IEEE Trans. BME, 16(2):116-122 (1969)] reported the development of a fully electronic analog system for immediate determination of oxygen saturation in whole blood. The system performed a double scale calculation, as described by Wood et al., using 650 nm red light and 800 nm infrared light. The system displayed an output as a logarithmic ratio after measurement of a saline blank and a whole blood sample.
The in vivo application of transmission oximeters has been impeded by non-specific absorption of red and infrared light, which causes major problems in the calibration of oximeters. In an attempt to overcome this problem, Merrick et al. [Continuous Non-invasive Measurements of Arterial Blood Oxygen Levels, Hewlett-Packard J., 28(2):2-9 (1976)] developed a multi-wavelength ear oximeter employing 8 suitably chosen wavelengths between 650 nm and 1050 nm to adequately resolve the difference in light absorption with sufficient accuracy to derive SaO.sub.2. This device basically extracted the necessary information from the spectral signature of the transmitted light signal, but still required hyperthermal arterialization of the capillary blood in the ear. The oximeter measured SaO.sub.2 within 95% confidence limits of .+-.4% when in the range of 65% to 100% saturation with a response time constant of 3 seconds. However, at saturation levels below 65%, the oximeter consistently under-estimated the SaO.sub.2.
Yoshiya et al. [Spectrophotometric Monitoring of Arterial Oxygen Saturation in the Fingertip, Med. & Biol. Eng. & Comput., 18:27-32 (1980)] introduced a pulse oximeter, recognizing that the pulsatile nature of arterial blood flow could be employed to circumvent two fundamental problems (nonspecific absorption and the arterialization requirement) of transmission oximetry. Previous transmission oximeters had measured the total signal of transillumination without discriminating between an oscillatory component caused by arterial blood changes and the non-oscillatory component caused by non-arterial blood and tissues. In pulse oximetry, the non-oscillatory component is electronically discarded and only the oscillatory component is utilized to determine SaO.sub.2.
For traditional, clinical applications, the conventional, transmission mode, pulse oximeter is one means of rapid, noninvasive, and continuous monitoring of peripheral tissue oxygenation. However, this technology is sorely inadequate for use at the scene of an emergency due to a minimum number of suitable measurement sites on the patient, the time required to affix sensors, the susceptibility to noise or vibration, patient motion, and the inability to operate through clothing.
Transmission mode pulse oximeters have other important limitations. Raised bilirubin concentration will cause under-reading of the true SaO.sub.2 saturation, as it absorbs light in the wavelengths used. A pulsating arteriolar vascular bed is required. The device will fail under conditions of cardiac arrest, placement of sensors distal to an inflated tourniquet or blood pressure cuff, intense vasoconstriction (due to chemical agents or hypothermia) or severe hypovolemia. A site for transillumination is required for sensor placement and motion artifact occurs when moving patients.
Polanyi et al. [New Reflection Oximeter, Rev. Sci. Instr., 31(4):401-403 (1960) and In Vivo Oximeter with Fast Dynamic Response, Rev. Sci. Instr., 33:1050-1054 (1962)] reported a device for in vivo measurements using fiberoptics mounted on a cardiac catheter. Subsequently, recognizing the utility of the reflectance approach for in-dwelling catheters, a number of workers developed or evaluated devices for in vivo reflectance oximetry. However, this work centered on developing invasive, catheter-based, devices.
U.S. Pat. No. 4,714,080 to Edgar, Jr. et al. concerns a non-invasive optical oximeter. A tissue sample is illuminated with light at two wavelengths. A photodetector senses light reflected by the tissue sample and produces an output signal indicating oxygen saturation.
In contrast to transmission oximetry, reflectance oximetry does not require transillumination and, thus, there is no limitation regarding measurement sites. Reflectance oximeters work quite well except at low oxygen saturations and abnormal hematocrits (except for recent devices incorporating hematocrit correction). However, reflectance oximeters of the past have required the direct exposure of tissue of a patient to the oximeter. The presence of any material, such as clothing or a protective wrap, between the reflectance oximeter and the tissue of the patient prevented such an oximeter from indicating tissue oxygenation.
A need exists for a device that instantaneously and noninvasively assesses tissue oxygenation of a patient without requiring the removal of clothing or protective wraps from the patient.